Off-HUD Payments

We are towards the very end of a vary protracted short sale process. The sellers bank has come back with an approval but said that they want 9,000 more and that it can’t show up on their
HUD-1. Is this illegal? If it is, can I use it’s illegality to pressure the seller’s bank into dropping the matter for fear of HUD investigation? If we weren’t 7 months and numerous fees into this I’d just walk, but it seems like a shame for it to fall apart because of sketchiness and incompetence.